The present invention relates, in general, to optical parametric generation, and more particularly, to a method and apparatus for broadly tuning optical parametric sources that produce single longitudinal mode energy over a wide variety of frequencies.
The conversion of light from one frequency to another in optical crystals using induced quadratic nonlinearities proceeds by a three-wave interaction process that has been described by John David Bierlein and Thurman Eugene Gier in U.S. Pat. No. 3,949,323. The conversion of light from one frequency to another using the quadratic nonlinearity term of the induced polarization in a medium having nonlinear optical properties gives rise to the phenomena of sum and difference frequency mixing. This is a catalytic process, since it occurs with essentially no exchange of energy between the optical crystal and the electromagnetic fields.
Energy conservation requires that the three frequencies involved, herein called the pump (.omega..sub.p) signal (.omega..sub.s) and idler (.omega..sub.i) frequencies meet the condition: EQU .omega..sub.p =.omega..sub.s +.omega..sub.i
For efficient energy conversion to occur, it is also required that the momentum or phase velocity of the interacting waves be matched while propagating through the nonlinear medium. This phase matching requirement is defined by the k vectors of the individual waves as: EQU .DELTA.k=k.sub.p -k.sub.s -k.sub.i
where .vertline.k.vertline.=.omega.*n(.omega.)/c, .DELTA.k is a measure of the phase mismatch, and n(.omega.) is the medium index of refraction for the wave at frequency .omega..
Phase matching is accomplished and the efficiency of conversion is maximized for .DELTA.k=0. This is typically achieved in nonlinear crystals by rotating the crystal to an angle at which the refractive indices for the waves is such that phase matching is achieved. Application of electric fields, changing the crystal temperature, or varying the relative propagation direction of the three interacting waves can also be used to achieve phase matching. When the k vectors of the three waves are not parallel, the process is termed noncollinear phase matching. The special case where all three waves are parallel is termed collinear phase matching.
Tunable pulsed optical parametric oscillators (OPOs) using nonlinear optical crystals have been extensively studied and developed since the mid 1960s. These early OPOs, however, typically had relatively broad linewidths, poor temporal and spatial beam qualities and produced relatively low powers and/or energies. In addition, the early OPOs that could provide single longitudinal modes had relatively many parts and were difficult to align.
It was also discovered that mode-hopping and mode beating of these tunable pulsed OPOs occurred as the OPO was tuned over it's entire tuning range. Since output beams that exhibit more than one longitudinal mode of the oscillator are undesirable for many applications, a tunable single longitudinal mode is preferred.
Accordingly, there is a need in the art to provide a simple pulsed OPO that can produce a tunable single longitudinal mode output over a broad range of frequencies at moderate powers for long periods of time.